Simultaneous flow visualization and local heat transfer measurements of two-phase flow in plate heat exchangers

Farraj, Abdel Rahman

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https://hdl.handle.net/2142/117633 Copy

Description

Title

Simultaneous flow visualization and local heat transfer measurements of two-phase flow in plate heat exchangers

Author(s)

Farraj, Abdel Rahman

Issue Date

2022-12-05

Director of Research (if dissertation) or Advisor (if thesis)

Hrnjak, Predrag S

Doctoral Committee Chair(s)

Hrnjak, Predrag S

Committee Member(s)

• Jacobi, Anthony
• Elbel, Stefan
• Kozlowski, Tomasz

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Plate heat exchanger
• local heat transfer
• flow regime visualization
• calibration
• experimental verification
• single-phase flow
• local heat transfer coefficient.

Abstract

Despite their wide applications in industry, heat transfer and pressure drop of two-phase flow in plate heat exchangers are not well characterized. Efforts from the air conditioner and refrigeration community have investigated the thermal-hydraulic performance of PHEs as evaporators or condensers. However, no refined general correlations in the public domain can account for all the different effects of geometrical parameters, working fluids, and operating conditions. This is especially true for the two-phase flow, as the heat transfer gains complexity as a result of the different flow regimes. The investigations described in the existing literature have either focused on measuring the heat transfer coefficient or separately visualizing the flow under simplified conditions. Most of the proposed heat transfer coefficients were based on the inlet/outlet values after considering the average heat transfer coefficient for complete evaporation. Some attempts have been focused on measuring the local thermal performance, but in these experiments, adjustments have been made to the tested plate heat exchanger. Therefore, the research presented here aims to develop a new experimental approach to characterize the two-phase flow in plate heat exchangers by combining local heat transfer measurements and flow visualization. In addition, this research aims to analyze the complete evaporation flow in the channel without any adjustments in distributer, geometry, or applied heat. Therefore, a commercial frame-and-plate heat exchanger was used to reflect the actuality of the flow. The number of plates was reduced to form three channels with R245fa flowing upward in the central channel while evaporating by two end channels in a counterflow configuration. A heat flux meter plate substituted one inner plate to collect the local thermal performance. The heat flux meter was built from two original plates measuring the inner wall temperature on each fluid side. A thermal infill of known thickness separated the two plates to calculate the heat flux. The other plates of the refrigerant channel were substituted with transparent plates to allow visual access, while two transparent plates were duplicated from the original plate. The measurements of the local heat transfer coefficient with the flow regime visualized explain the behavior of the flow boiling at different areas in the plate. After fabricating the heat flux meter, careful calibration steps were considered to achieve accurate heat transfer results. The conductive heat through the HFM was compared and verified with the heat transferred in the streams for single- and two-phase flow. The mean of the heat transfer coefficient values was compared with another calculating approach, using the overall heat transfer coefficient. In addition, the verification of the heat transfer coefficient reading in the evaporating flow of R245fa was compared with other correlations from literature and data obtained from another brazed plate heat exchanger used in a separate heat pump system. These steps were essential to consider before showing results to validate the accurate reading of the HFM. The thermal-hydraulic performance of single-phase heat transfer is first measured in the PHEs. Water is a working fluid in a vertical countercurrent flow two-channel arrangement. The advantage of the HFM showed the distribution of temperature walls on two sides with how the thermal resistance of each stream influenced different locations. ANSYS Fluent's (2020 R1) commercial code was used for computational fluid dynamics simulation with the realizable k-ε turbulence model. The simulation was used mainly to predict the velocity distribution for the single-phase in the domain. After applying the same conditions to the simulated domain, the outlet temperature from the simulation was verified experimentally. Finally, the evaporating flow was investigated by simultaneously measuring the local heat transfer coefficient and visualization. Results of R245fa flow boiling are presented here, as they are impacted by changes in mass flux, heat flux, and inlet vapor quality. Different flow regimes were identified, including the liquid pool, irregular bubbly flow, and liquid dry-out zone. In the fluid dry-out regime, liquid retention was found around the contact points. The prevailing liquid pool under various inlet flow regimes indicated that it is not a result of inlet distribution and mass flow rate but rather a result of liquid separation in the plate channel. Nucleation sites at different locations along the plate were also observed, starting from the liquid pool region. The motion of the bubbles indicated that the liquid pool is rather stagnant, and buoyancy is the primary driving force. Both the number and activeness of nucleation sites increased with heat flux. Nucleation sites were also found around the contact points, where liquid is accumulated even near the dry-out region.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

© 2022 Abdel-Rahman Farraj

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